Droplet impact dynamics: Effect of varying substrate temperature, roughness, and droplet velocity

An experimental apparatus has been designed and built to photograph high-speed impact of small liquid and molten metal droplets on surfaces of varying temperature and roughness. Two different droplet generators were used in order to produce uniform water droplets (0.55 mm) and molten tin droplets (0.6 mm). A stainless steel substrate was mounted on the end of a rotating flywheel, giving linear velocities of up to 50 m/s. Different stages of droplet impact were photographed by synchronizing the ejection of a single droplet with the position of the rotating flywheel and triggering of a camera.; Results are presented first for impact of water droplets on two stainless steel substrates with an average roughness of 0.03 or 0.23 μm and an impact velocity of 10–50 m/s. A mathematical model (Kim et al., 2000), based on linear Rayleigh-Taylor instability theory, was used to predict the wavelength of the fastest growing perturbation around a spreading droplet. The corresponding wave number agreed reasonably well with the number of fingers around the droplet.; For the molten tin droplet impact droplet impact velocity (10–40 m/s) and the substrate temperature (120–240°C) were varied. A critical temperature range (160–180°C) was found about which the dynamics of splashing transformed from a fragmentation splashing to a smooth splashing. Finally different stages of tin coating formation were photographed on stainless steel surfaces for two roughness: R_a = 0.05 and 2 μm, and three substrate temperatures: T_s = 120, 180, and 220°C. The impact velocity was held constant at 30 m/s. The maximum deposition efficiency was achieved at a substrate temperature of about 160°C. For T_s < 160°C the deposition efficiency was higher on a rough surface (R _a = 2 μm) than on a smooth surface (R_a = 0.05 μm), since splats did not adhere well to the smooth surface. For T_s ≥ 160°C the deposition efficiency was higher on a smooth surface (R_a = 0.05 μm) than on a rough surface (R_a = 2 μm), since splats splashed less on the smooth surface.